1. Field of the Invention
The invention relates to glass sealants, especially for sealing housings made of aluminum oxide ceramics. Such glass sealants are also designated ceramic dual-inline package (CERDIP) solder. Heavy demands are made on these glass sealants when they are used to encapsulate large-scale integrated circuits. The sealants must have a very low melting temperature, a low dielectric constant (DC), a low loss angle (tan delta), excellent chemical resistance, low .alpha.-particle emission, high mechanical strength, good diathermic adaptation to the housing material and the conductor material, as well as good thermal shock strength.
2. Description of Related Art
Glass sealants especially for integrated circuits are well known in the art. From DE-PS 32 39 039, a glass sealant is known, which consists of a lead-borate glass containing a low-melting bismuth oxide and 8-12% by weight of .beta.-eucryptite. However, the strength of this glass sealant is mediocre; it tends to sustain microcracks from heat shock, which can result in damage to the integrity of the seal.
In DE-OS 33 29 102, a glass sealant is described, which consists of 50-80% by weight of a low-melting solder glass powder of a PbO--B.sub.2 O.sub.3 system, 1-35% by weight of a ceramic composition of zinc oxide and silicon dioxide, as well as 1-45% by weight of tin dioxide. This glass sealant has a relatively high density which, to a user of the sealant, appears as a comparatively high consumption (in terms of weight). The electrical properties, such as the dielectric constant and loss angle, are not satisfactory so that problems can occur at high signal velocities. Further, this sealant has unsatisfactory chemical resistance to acids used in an electroplating tin unit in which the electric wires projecting from the sealed housing are tinned. In this unit, undissolved sulfates can form on the free glass sealant surfaces. This leads to difficulties since during electroplating the tin has a tendency to precipitate on the precipitated sulfate, which disturbs or interrupts the normal operation of the encapsulated integrated circuit. The ceramic composition containing silicon dioxide and tin oxide consists mainly of willemite, which has low mechanical strength. In thermal shock tests, it has been found that microcracks are formed in this material.
In DE-OS 33 43 570, a glass sealant is described, comprising 75-50% by volume of a low-melting glass powder with a content of lead oxide as the main component, 20-45% by volume of ceramic powder, especially cordierite, as well as 5-30% by volume of a pulverized solid solution of titanium dioxide and tin dioxide. Also, this glass sealant has a relatively high density and dielectric constant. A high dielectric constant at high signal frequencies can lead to disturbances in large-scale integrated circuits.
From DE-OS 35 09 955, a glass sealant is known, which consists of 50-80% by weight of a solder glass powder made from a lead oxide B.sub.2 O.sub.3 system, 1-35% by weight of an inert zinc powder, substantial willemite, and 1-35% by weight of synthetic zirconia which, with the exclusion of radioactive substances, is artificially prepared. This glass sealant is relatively expensive due to the synthetic zirconia which requires repeated recrystallization of zirconium salts. Also, this sealant has the disadvantages that the loss angle tan delta and the density are relatively high.
An object of the invention is to provide an improved glass sealant, especially for sealing aluminum oxide ceramic housings for integrated circuits, which at very low melting temperatures has a high mechanical strength, a good sealing capability, high chemical resistance, as well as favorable electrical properties, and especially a low dielectric constant.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.